As a result, they say the Chancellor will be guilty of “bad faith” if it does cut the allowance in his Autumn Statement today.

A Treasury document issued when the Government decided on a previous cut in the annual limit made an “implied commitment” not to reduce the allowance below the current level of £50,000 until at least 2016.

It also suggested that any future changes in the limit would be upwards, in line with inflation.

The document, dated October 2010, said: “The Government has decided that the level of the AA [annual allowance] will be set at £50,000 from April 2011 ... Beyond the forecast period to 2015-16 the Government will consider options for indexing the level of the AA.”

Reducing the annual allowance will expose many public-sector workers to tax bills. This is because their generous final salary pension schemes are treated for tax purposes as involving large annual contributions, particularly if the employee receives a pay rise through promotion, for example.

Ros Altmann, the director-general of Saga, the over-50s’ group, said: “The Government implied that the annual allowance would remain at £50,000 until 2016. Although the Chancellor can easily say that the public finances have worsened since 2010, he should acknowledge that he has broken an implied commitment if he does indeed announce a cut in the allowance in the Autumn Statement.

“This document clearly made an implied commitment that the annual allowance would be kept at £50,000 and then potentially increased in line with inflation.”

One senior adviser to pension schemes agreed. Chris Noon, a partner at Hymans Robertson, the actuary, said: “I think the Government would be guilty of bad faith in reducing the annual allowance so soon after the setting it at £50,000.”

He added: “During the consultation process the pensions industry asked for clarity on how the annual allowance might change in the future. Without clarity it is difficult for individuals and employers to design retirement plans because pensions are a long-term commitment. Even though there was ambiguity about what might happen after five years, the clarity that it [the annual allowance] was fixed for five years was welcome and has been used by individuals in their planning.”

Other experts said the wording in the document did not constitute a cast-iron commitment, however. Malcolm McLean of Barnett Waddingham, another firm of actuaries, said: “This is not an explicit commitment not to cut the annual allowance again. Even it was, it’s not a legal document and wouldn’t stop the Government saying that circumstances have changed and the requirements of the economy are such that they have to look at everything afresh, take tough decisions and so on.”

But Ms Altmann said the Government “has form” when it comes to broken commitments on tax relief. The Telegraph disclosed in April that the imposition of the “granny tax” – a freeze in the personal allowance for pensioners – broke a commitment in Budget documents to increase it in line with inflation. “This was another occasion when the Government refused to acknowledge that it was breaking a commitment,” Ms Altmann said.

Alan Morahan of Punter Southall, the pensions consultancy, said: “The Government needs to be mindful of the law of unintended consequences. Were the Chancellor to reduce the annual allowance to as low as £30,000 even relatively moderate earners could be affected.”

A spokesman for the Treasury said she wasn’t able to comment on pre-Autumn Statement speculation.